There are many ways to reconfigure power from one form into another. Reconfiguration from a direct current source into an alternating current source is the most common form. Other ways include converting a DC power source having a varying level into a fixed level DC source, converting alternating current into direct current, and changing the level or frequency of alternating current.
There has developed a need for power reconfiguring systems for independent power generating systems, such as in locations where there is no electrical power available from a public utility, where installations exist which must have an uninterruptible power source, and in locations served by utility power where off-peak utility rates are lower, or where load levelling can reduce utility demand charges.
Electricity for an independent power system can be generated by several methods. Photovoltaic panels (PV solar cells), windmills, waterwheels, and diesel and gasoline generators can all be used. Photovoltaic is becoming the most popular way to generate electricity because it is nonpolluting, does not require refueling, has no moving parts to wear out, and the cost of PV solar cells continues to drop, making PV systems more cost effective. Further, the need for nonpolluting energy sources has dramatically increased in the last few years, especially since the occurrence of recent nuclear accidents around the world and the continuing rise of the cost of electricity generated by public and private utilities.
Photovoltaic panels produce level-varying direct current (DC) electricity. DC power can be stored in batteries for later use. When batteries are drained, the power they release is DC. However, all common household and business machines operate on AC 60 cycle power or 50 cycle power. The user who desires to use existing equipment with standard, ordinary AC wiring, must have a source of AC power, and must convert the DC power into AC. The device to perform simply the conversion of DC power to AC power is commonly called an inverter.
One drawback which has retarded the use of small photovoltaic power systems is the lack of a reliable apparatus to reconfigure the power into clean, undistorted AC power efficiently. Many developments have occurred in the last 20 years as a result of the increasing need to provide independent power sources. As a result, many circuits have been developed which attempt to achieve, for varying applications, the conversion or reconfiguration of power from one form into another. The following is a brief description of some of the many patents which have recently been issued in this area.
U.S. Pat. No. 3,579,081 issued May 18, 1971 to Bates discloses a low frequency sine wave generator circuit having three bridge circuits with each bridge circuit having four switches to generate a voltage pattern to produce a step-wise quasi-sine wave for a load. U.S. Pat. No. 3,581,212 issued on May 25, 1971 to McMurray discloses a fast response stepped-wave switching power converter circuit which includes the use of a feedback signal to offset a reference signal to generate a target signal. The target signal passes through an analog to digital converter and control logic for producing gating signals for the individual bridge switches to keep the output value within a range of values. U.S. Pat. No. 3,614,589 issued Oct. 19, 1971 to Ireland et al. discloses a sine wave static inverter using a logic-based pulse generator and stepped-wave generators with a harmonic analyzer to convert DC power into quasi-sine wave power.
U.S. Pat. No. 3,648,149 issued Mar. 11, 1971 to Brown et al. discloses a polyphase inverter circuit which uses power borrowed from a transformer to achieve polyphase stepped voltages. U.S. Pat. No. 3,691,449 issued Sept. 12, 1972 to Feltman discloses a static inverter having automatic means for providing a highly regulated AC output despite changes in the input DC level. This system uses an integrator charged by input DC level via a static converter and generates inverse signals to separately drive transistor switches of the bridge. U.S. Pat. No. 3,750,004 issued July 31, 1973 to Walker discloses an instantaneous current control for static inverters. This device provides current limiting of three phase AC output from DC input. Control is by adjusting the relative phases of the inverters. U.S. Pat. No. 3,767,996 issued Oct. 23, 1973 to Bates discloses a low frequency multi-phase sine wave generator circuit which uses major and minor three-bridge networks with separate switch control to generate three phase output from DC input.
U.S. Pat. No. 3,867,643 issued Feb. 18, 1975 to Baker et al. discloses an electric power converter having a seven-stage bridge network with logic control for operating each switch. U.S. Pat. No. 4,032,832 issued June 28, 1977 to Miller discloses a DC to AC inverter with unregulated input and regulated output. Regulation is achieved by sensing the output amplitude and comparing it with a reference to shift the phase between bridge outputs. The bridges are controlled by a signal applied to each bridge half with the use of transformers and reverse windings to achieve inverse signals applied to the transistor switches. U.S. Pat. No. 4,131,936 issued Dec. 26, 1978 to Gemp discloses a digital delaying device for use in a symmetrical phase shifting apparatus having a harmonic neutralized system. This system has a four-stage square wave switching circuit which uses phase shifting of a memory using relative delays between reading and writing of data from the memory to generate three phase power.
U.S. Pat. No. 4,159,515 issued June 26, 1979 to Walkowiak discloses an inverter control system which provides controlled start up and shutdown based on low input voltage, high output current and other factors. U.S. Pat. No. 4,161,771 issued July 17, 1979 to Bates discloses an inverter ripple regulator which senses the ripple in an output and uses an internally generated signal to produce an error signal which controls a buck-boost circuit in series with the inverters. U.S. Pat. No. 4,200,908 issued Apr. 29, 1980 to Kalinichenko et al. discloses a device for digital control of a polyphase thyristor-pulse. This circuit uses a clock pulse counter to generate pulse trains to control polyphase thyristor-pulse converters and uses time-shifted pulse trains to vary the phase.
U.S. Pat. No. 4,240,135 issued Dec. 16, 1980 to Schaefer, III discloses a regulated cycloconverter circuit which uses a regulator to add a rectified voltage to the battery source and a microcomputer to control timing of a lower voltage output. U.S. Pat. No. 4,270,163 issued May 26, 1981 to Baker discloses a bridge converter circuit which applies positive and negative input voltages and respective switching of these so that a step-wise alternating current output varying .+-.2 times the input voltage is achieved. U.S. Pat. No. 4,370,702 issued Jan. 25, 1983 to Shuey et al. discloses microprocessor control of inverter power transistors with DC content correction. This system produces three phase power and has current limit control and pulse pattern generator using a single bridge per phase with two switches for each bridge with individual switch control.
U.S. Pat. No. 4,399,499 issued Aug. 16, 1983 to Butcher et al. discloses a bi-lateral four quadrant power converter which uses direct current to produce alternating or direct current output. A four-switch bridge is controlled using complementary signals generated by inverters. U.S. Pat. No. 4,476,520 issued Oct. 9, 1984 to Gallemore discloses a computer controlled synthetic waveform generator which uses an EMF ring of charge-storage elements with computer controlled switching to achieve the alternating current output.
Most of these prior art devices are actually square wave inverters which produce modified square wave or quasi-sine wave outputs. These outputs are not very close to a smoothed sine wave and thus are frequently "dirty" in that they are unsuitable to operate equipment which provides an inductive load. Although some of these are able to achieve a relatively high efficiency, this is typically under the most favorable conditions while the unit is running at or near its capacity and usually with a resistive load. An inverter was developed by the inventors of this invention and disclosed at a 1985 trade show. The apparatus was operated in a "black box" form with the output displayed on an oscilloscope screen. This device was an improvement over other known devices in that it provided a three bridge circuit with the relative voltage transformation from each bridge being related to the others by a factor of three. Thus, the third bridge produced power which was one-ninth the power level of the main bridge. The bridges were operated in half bridge segments with a single control signal directed to each half bridge with series-coupled switches being operated by actively controlling one of the switches. The other switch then operated by default, taking the reverse operation of the actively controlled switch. Although this device provided a substantial improvement in the "cleanness" of the output sine wave, it was not as ultimately efficient and controllable as is desired. Further, it did not have safety shutdown features which would be necessary in an actual use situation.